This invention relates to a color cathode ray tube, and more particularly to a blackened surface construction of its metallic members whose main constitutent is iron, such as the shadow mask assembly or the inner shield.
A color cathode ray tube has an envelope provided with a panel having a phosphor screen essentially consisting of a mosaic of phosphors emitting red, green, and blue light, a neck housing an electron gun, and a funnel that connects the panel and the neck.
The electron beams emitted from the electron gun are selectively allowed to pass through to the screen by arranging, facing the phosphor screen, a shadow mask provided with a large number of apertures.
However, when these electron beams are subjected to the effect of an external magnetic field such as the earth's magnetic field, in particular in the vicinity of the shadow mask, the trajectories of the electron beams are disturbed, resulting in landing errors on the phosphor screen. Further problems are caused by the fact that the proportion of the electron beams which passes through the apertures of the shadow mask is only about 20%. The rest of the electrons in the beams, i.e. the electrons that do not pass through the apertures, are elastically reflected by the shadow mask, causing heating of the shadow mask and undesired emission of light from the phosphor screen.
To solve these problems use is made of an inner shield. The inner shield is fixed on the funnel side of the shadow mask. The material of the inner shield is required to have high permeability, to be electrically conductive, to have good forming properties, high mechanical strength and not to develop rust during the manufacturing process. Soft steel sheet is iron is therefore usually used for this purpose.
Regarding the reflective scattering of the electron beam and heating of the shadow mask, to suppress the radiation of secondary electrons and to confer black body radiation characteristics, the surfaces of the shadow mask assembly and/or inner shield are subjected to oxidative treatment to form a blackened film. This blackened film also serves to prevent rust.
On the other hand, this blackened film itself gives rise to some problems such as poor withstand voltage characteristics and/or blockage of the apertures of the shadow mask caused by peeling of this blackened film, areas of lowered contrast due to insufficient suppression of scattered electrons, or reduced emission life due to production of gaseous impurities when the mask is struck by the electron beams.
As a substitute for the blackened film of iron oxide, Japanese Patent Application Laid Open No. 50-15766 discloses an attempt to obtain a black Fe-Al alloy surface by attaching aluminium to the surface of an iron inner shield and dispersing the aluminium in the surface of the inner shield by subjecting it to heat treatment. With this set-up, there is some improvement in respect of the problem of film peeling, but there is no improvement in respect of emission life, which is reduced by gaseous impurities generated by the impingement of the electron beams.